Field of the Invention
The present invention relates to a transaxle system for a vehicle, especially, for a working vehicle equipped with a working device, e.g., a lawn mower equipped with a mower unit, wherein the vehicle transaxle system includes right and left transaxle units each of which supports a right or left corresponding single axle.
Related Art
As disclosed in U.S. Pat. No. 6,385,971, there is a well-known conventional zero-turn type vehicle transaxle system for a working vehicle, such as a lawn mower, for enabling zero-turn of the vehicle, thereby ensuring a satisfactory working efficiency. The transaxle system includes right and left hydraulic transaxle units each of which supports a right or left corresponding single axle. Each of the transaxle units includes a hydrostatic transmission for driving the corresponding axle, and the hydrostatic transmission includes hydraulic pump and motor fluidly connected to each other through a closed fluid circuit.
In each of the transaxle units of the conventional transaxle system, the hydraulic pump has a variable displacement, and the hydraulic motor has a fixed displacement, so that the output rotary speed of the hydraulic motor depends on controlling of the displacement of the hydraulic pump. Even if the maximum displacement of the hydraulic pump is sufficient to ensure a high torque traveling of the vehicle at work, it may be insufficient to ensure a required efficient power transmission for normal high-speed traveling of the vehicle without work. Therefore, conventionally, the transaxle unit further includes a sub speed-changing transmission, such as a gear transmission, that has at least two low-and-high speed drive trains and is interposed between the hydraulic motor and the axle. When the vehicle without work normally travels at a high speed, the high speed drive train of the sub speed-changing transmission is selected for driving the axle, so as to compensate for the lack of efficiency of power transmission by the hydrostatic transmission. However, the conventional transaxle unit is expanded in size and is expensive because of the arrangement of the sub speed-changing transmission.
Further, a sub speed-changing manipulator for operating the sub speed-changing transmission may be wrongly operated during traveling of the vehicle. For example, although a skilled operator can shift the sub speed-changing manipulator to a high speed setting position at an appropriate timing during traveling of the vehicle at work so as to improve the working efficiency, the operation of the sub speed-changing manipulator is too difficult for an unskilled operator to judge a timing for shifting the sub speed-changing manipulator to the high speed setting position, so that the unskilled operator may shift the sub speed-changing manipulator to the high speed setting position at a wrong timing so as to unexpectedly accelerate the vehicle and to spoil the stability of work. Further, if the vehicle frequently moves from one work area to another work area, a drive train for the working device must be clutched on and off on every movement of the vehicle between work areas. The high speed level of the sub speed-changing transmission may be desired during the movement of the vehicle between work areas. However, to perform both the clutch operation and the sub speed-changing operation is difficult and likely to unexpectedly keep the driving of the working device during the movement of the vehicle setting the high speed level of the sub speed-changing transmission, thereby causing power loss and spoiling the stability of work. Further, the sub-speed changing operation during traveling of the vehicle causes sudden change of traveling speed of the vehicle. Especially, the zero-turn type vehicle may be provided with a pair of right and left main speed-changing manipulators for controlling the respective hydraulic pumps of the respective right and left transaxle units. In this case, the manipulation of the right and left main speed-changing manipulators for the main speed-changing and left-and-right turning of the vehicle is complicated and likely to cause the wrong sub speed-changing operation. Therefore, the vehicle is desired to avoid unexpected speed change even if the sub speed-changing manipulator is wrongly operated.
Therefore, it is conceivable that the hydraulic motors of the respective transaxle units are configured to have respective displacement control means, e.g., movable swash plates, for changing their respective displacements so that the displacement control means of the hydraulic motors in the right and left transaxle units are operable to select a traveling mode of a vehicle between a low speed traveling mode for working traveling of the vehicle and a high speed traveling mode for on-road traveling of the vehicle, thereby eliminating additional sub speed-changing gear trains for changing speed ratios of axles to respective hydraulic motors so as to minimize the right and left transaxle units.
However, if the displacement control means of the hydraulic motors in the right and left transaxle units are movable swash plates, for example, the movable swash plates of the respective hydraulic motors have to be simultaneously operated in consideration of tilting directions of the movable swash plates of the hydraulic motors. Especially, if the hydraulic motors in the right and left transaxle units have rotary axes parallel or coaxial to respective axles, it should be considered that tilting directions of the movable swash plates between respective large tilt angles (defining large displacements for low speed level) and respective small tilt angles (defining small displacements for high speed level) are laterally opposite each other. A simple and economic operation system is desirable to enable such a simultaneous operation of the displacement control means of the hydraulic motors of the right and left transaxle units coping with the opposite rotational directions of the displacement control means, such as movable swash plates.
Further, if the movable swash plates of the hydraulic motors in the right and left transaxle units can be operated for accelerating a vehicle by manipulating not an additional sub speed-changing manipulator but the right and left main speed-changing manipulators that are frequently manipulated for controlling the respective hydraulic pumps in fluid delivery amount and direction during traveling of the vehicle, it is very convenient for an operator skilled in manipulating the right and left main speed-changing manipulators, and an operation system for controlling movable swash plates of the hydraulic pumps and motors in the right and left transaxle units can be entirely minimized. However, such an operation system should be configured so that movement of the movable swash plates of the hydraulic motors during traveling of the vehicle does not cause a sudden speed change that reduces the stability of the vehicle while traveling.